It is an important problem in agriculture to diminish or eliminate injury by continuous cropping. Especially in countries and districts having a limited area of cultivated land, crops must be repeatedly planted through the year and, therefore, continuous cropping is unavoidable.
Under these conditions, biological factors associated with the soil interfere with the growth of various crops such as vegetables, legumes, potatoes, strawberry, tobacco, devil's-tongue, chrysanthemum, carnation, stock, mulberry, apple tree, upland rice, and the like, so that farmers suffer heavy losses. Moreover, the situation goes on getting worse from year to year.
Conventionally, the control of plant diseases and, in particular, soil-borne diseases is very difficult and there is a continuing demand for the development of excellent fungicides. By way of example, the damage due to clubroot disease of Brassica spp. shows a tendency to increase from year to year. That is, important vegetables (such as cabbages, Chinese cabbages, turnip, and the like) indispensable to the dietary life of men are being considerably damaged by this clubroot disease.
At present, various attempts are being made to control soil-borne diseases by means of fungicides. However, commercially available fungicides fail to produce desirable results and cannot be regarded as preferable from a practical point of view. More specifically, in the existing circumstances, none of the commerically available soil fungicides are satisfactory because they have the disadvantages of failing to produce their fungicidal effect unless used at high concentrations, tending to remain in the crop plants or the soil, having very high toxicity to men and beasts, tending to exert phytotoxicity on the crop, and/or having an irritant or an unpleasant odor.
For example, mercury compounds have very high toxicity and volatile substances such as methyl bromide and chloropicrin tend to give off a toxic and irritant gas, so that they may give rise to the problem of environmental pollution. Pentachloronitrobenzene (PCNB) fails to effectively control clubroot disease of Brassica spp., unless applied to the crop in large amounts. Moreover, this compound can hardly be decomposed owing to its stable chemical structure, thus involving a possibility of soil pollution. Furthermore, PCNB tends to be accompanied by hexachlorobenzene, as an impurity, which is difficult to separate during the manufacture of PCNB. It is well known that this compound is undesirable because it remains in the soil for a long period of time and also has high toxicity. Besides, methyl isothiocyanate requires repeated degassing operations in order to avoid its phytotoxicity. Nevertheless, the possibility of phytotoxicity cannot completely be eliminated.
Benzene sulfonanilide and its derivatives are known for a long time, but only a little is known about analogous compounds having their aromatic ring substituted by a nitro group. Japanese Patent Publication No. 41638/'71 discloses 2-nitrobenzene sulfone-2,4-dichloroanilide which is effective in controlling citrus canker. Japanese Patent Publication No. 15119/'72 discloses three nitro-substituted benzene sulfonanilides (i.e., 3-nitrobenzene sulfonanilide, 3-nitrobenzene sulfone-4-chloroanilide, and 3-nitrobenzene sulfone-3,4-dichloroanilide), which are described as being effective in controlling brown leaf spot and pellicularia disease of rice. Japanese Patent Laid-Open No. 31655/'82 discloses 2-nitrobenzene sulfone-2,6-diethylanilide, which is described as being effective in controlling rice blast.
As described above, a number of nitro-substituted benzene sulfonanilides are known in the prior art. However, nothing is known about the nitro-substituted benzene sulfonanilides having both aromatic rings substituted by nitro groups. Moreover, neither experiment on the ability of benzene sulfonanilide and its derivatives to control soil-borne diseases, nor literature disclosing their effectiveness in controlling soil-borne diseases can be found.